1. Field of the Invention
The present invention relates generally to micro-machined back-flush injectors for gas chromatography. The present invention also relates to methods for manufacturing and operating micro-machined back-flush injectors.
2. Description of the Related Art
FIG. 1 illustrates a back-flush injector 10 according to the related art. The injector 10 includes a carrier gas inlet 20 connected to a main carrier gas loop 30 that is terminated at a fore-flush valve 35.
Off-shooting from the main carrier gas loop 30 is a reference column loop 40 that terminates at a reference column inlet 50. Also, off-shooting from the main carrier gas loop 30 is a pre-column back-flush loop 60 that terminates at a back-flush valve 70. A gas chromatography reference column (not shown) is positioned external to the injector 10 and operably connected to the reference column inlet 50. The reference column, typically used in conjunction with a thermal conductivity detector (not shown), enhances the detector signal and the overall sensitivity of the gas chromatography system.
The back-flush valve 70 is connected to an analytical column inlet channel 80 and a pre-column outlet channel 85. The analytical column inlet channel 80 leads to a gas chromatography analytical column (not shown) that is positioned externally to the injector 10. The pre-column outlet channel 85 leads to a pre-column (not shown) that will be discussed below.
A sample inlet 90 is also illustrated in FIG. 1. The sample inlet 90 is connected to an inlet channel 100 that, in turn, is connected to a sample valve 110. The sample valve 110 connects the inlet channel 100 to a dead volume channel 120 that extends to an injection valve 130.
One function of the injection valve 130 is to control flow between a pre-column inlet channel 135, that connects to the pre-column discussed above, and a fixed sample loop 140, that extends to the fore-flush valve 35. The fore-flush valve 35 regulates flow between the main carrier gas loop 30, the fixed sample loop 140, and a sample chamber 150. The back-flush valve 70 controls flow from the pre-column back-flush loop 60 into the analytical column inlet channel 80 and the pre-column outlet channel 85. The functions of these valves will be elaborated upon further when the operation of the injector 10 is discussed.
The sample chamber 150 terminates at a sample chamber outlet 160 that itself is connected to a switch solenoid 170, which is external to the injector 10. The switch solenoid 170 can either be opened to a carrier gas pressure source 180 or a pump 190 that leads to a vent 200. The pressure of gas in the carrier gas pressure source 180 is approximately the same as the pressure of the gas at the carrier gas inlet 20. The carrier gas pressure source 180, when allowed by the switch solenoid 170 to be connected to the sample chamber outlet 160, delivers carrier gas into the injector 10.
During gas chromatography analysis, a carrier gas at a regulated gas pressure is delivered by an outside source to the injector 10 through the gas carrier inlet 20. This carrier gas fills the main carrier gas loop 30, the reference column loop 40 and the pre-column back-flush loop 60. Carrier gas from the same outside source is also delivered to the carrier gas pressure source 180.
During operation, the injector 10 injects a gaseous sample to be analyzed via gas chromatography through the pre-column and analytical column discussed above. In order to properly inject the sample, the injector 10 uses five stages of operation. These stages include sampling, dwelling, sample compression, injection, and back-flushing.
During the operation of gas chromatograph and of the injector 10, a carrier gas such as, but not limited to, helium, hydrogen and argon, is delivered into the injector 10 through the carrier gas inlet 20 and fills the main carrier gas loop 30, the reference column loop 40 and the pre-column back-flush loop 60. The fore-flush valve 35 does not allow the carrier gas to flow into the fixed sample loop 140 or the sample chamber 150. The reference column inlet 50 allows some carrier gas to flow into the reference column. The carrier gas that enters the reference column does not return to the injector 10.
The back-flush valve 70 is also normally open during the idling stage (before the sample is introduced into the injector 10) and allows the carrier gas in the pre-column back-flush loop 60 to enter and fill the analytical column inlet channel 80 and the pre-column outlet channel 85. However, whether the back-flush carrier gas can travel into the fixed sample loop 140 is dependent on the status of the injection valve 130. When the injection valve 130 is open to the pre-column inlet channel 135, the carrier gas can then be delivered to the fixed sample loop 140 and the sample chamber 150. This flow is known as back-flushing.
The injector 10 can be set to allow back-flushing in the idling stage or can be set to not conduct back-flushing in order to reduce the consumption of the carrier gas. The carrier gas flow that passes through the analytical column inlet channel 80 proceeds to enter the analytical column, passes the detector (not shown), and does not return to the injector 10.
During the sampling stage, the sample valve 110 is opened and the pump 190 starts. Alternately, the pump 190 can be started earlier and the sample valve 110 can be opened subsequently. As another alternative, if the sample stream has a positive pressure, use of the pump 190 may not be needed.
Regardless of the alternative chosen, an inflow of gaseous sample from the sample inlet 90 enters and fills the inlet channel 100, passes through the sample valve 110 and fills the dead volume channel 120. The injection valve 130 allows the sample to fill the fixed sample loop 140 but does not allow flow of the sample into the pre-column inlet channel 135.
After the gaseous sample has moved through the fixed sample loop 140, it does not enter into the main carrier gas loop 30 because the fore-flush valve 35 is closed to this path. The sample can only travel into the sample chamber 150 and exits the injector 10 via the sample chamber outlet 160. Further, because the switch solenoid 170 is opened to the pump 190 during the sampling stage, the sample then travels through the pump 190 and exits the gas chromatographic instrument via the vent 200.
After the sampling stage, the sample valve 110 closes and the pump 190 stops drawing the sample into the injector 10. After approximately 100-500 milliseconds, the sample pressure in the fixed sample loop 140 and sample chamber 150 are set to be in equilibrium with the ambient pressure. This is known as the dwelling stage. Sample compression then follows.
During the compression stage, the switch solenoid 170 is actuated to open to the carrier gas pressure source 180 and a stream of carrier gas is delivered to the sample chamber 150 via the sample chamber outlet 160. Since the carrier gas has a higher pressure than the sample which has been set to be at ambient pressure during the dwelling stage, the carrier gas compresses the sample toward the fore-flush valve 35, the fixed sample loop 140, the injection valve 130, the dead volume channel 120, and the sample valve 110. Furthermore, during the compression stage, the fore-flush valve 35 does not allow the compressing sample to enter the main carrier gas loop 30.
During the injection stage, the injection valve 130 allows flow of the sample into the pre-column inlet channel 135. Also, the fore-flush valve 35 allows carrier gas from the carrier gas inlet 20 to travel from the main carrier gas loop 30 into the fixed sample loop 140 and sample chamber 150. However, since carrier gas from the carrier gas pressure source 180 is still compressing the sample, the only direction in which the carrier gas from the main carrier gas loop 30 can move is in one which forces the sample that was in the fixed sample loop 140 to enter the pre-column inlet channel 135 and, ultimately, the pre-column.
Also, during injection, the back-flush valve 70 closes and stops the back-flushing carrier gas in the pre-column back-flush loop 60 from entering into the analytical column inlet channel 80 and the pre-column outlet channel 85. This reduces resistance to the injection stream from the fore-flush valve 35 and the main carrier gas loop 30.
After the sample has entered and traveled through the pre-column, the sample re-enters the injector 10 through the pre-column outlet channel 85. Because the back-flush valve 70, during the injection stage, is positioned to allow the sample to flow from the pre-column outlet channel 85 to the analytical column inlet channel 80, the sample continues into the analytical column where the gas chromatographic analysis is conducted.
The above-described injection or fore-flushing stage typically takes several seconds to finish, depending on the particular gas chromatographic analysis undertaken. According to one type of analysis, all components of a sample to be analyzed are moved by the carrier gas towards the analytical column. However, during the movement of the components in the pre-column, some components may travel faster and some may be slower. Hence, the injection or fore-flushing time is selected to allow those components that are important to the analysis to move into the analytical column while leaving behind unimportant components in the pre-column.
During the back-flushing stage, which follows the fore-flushing stage, the unimportant components are purged away from the injector 10 so that they do not interfere with the analysis. In order to properly back-flush or xe2x80x9cpurgexe2x80x9d all residual sample components in the pre-column from the injector 10, the back-flush valve 70 is opened to allow carrier gas from the pre-column back-flush loop 60 to flow into both the analytical column inlet channel 80 and the pre-column outlet channel 85. This causes carrier gas from the carrier gas inlet 20 to back-flush the pre-column on one hand, and to continue to move the components of interest into the analytical column, through the analytical column and towards the detector.
Once the back-flushing carrier gas passes through the pre-column, the carrier gas travels through the pre-column inlet channel 135 and flows out of the injection valve 130, through the fixed sample loop 140, through the fore-flush valve 35 and into the sample chamber 150. Because the switch solenoid 170 is open to the pump 190 during the back-flushing stage, the back-flushing carrier gas and any residual sample pushed by the carrier gas is released through the vent 200.
As can be seen from FIG. 1, a short-coming of the related art injector 10 illustrated has to do with the fact that there is sample trapped in the dead volume channel 120 during the injection process. To understand the problem that the trapped sample presents, one must take into account that the injection carrier gas from the fore-flush valve 35 only takes a small fraction of a second (10-100 millisecond) to move all sample in the fixed sample loop 140 into the pre-column inlet channel 135. The rest of the injection time or fore-flushing is supposed to have only xe2x80x98purexe2x80x99 carrier gas flowing.
However, as there is no physical partition between the dead volume channel 120 and the fixed sample loop 140, the sample in the dead volume channel 120 continuously diffuses into the moving carrier gas stream and get xe2x80x98injectedxe2x80x99, trace amount by trace amount, into the pre-column and the rest of the device. Since sample components with higher volatility and concentration diffuse faster, the chromatograms of these components are interfered with and unwanted shoulders 33 are found on the gas chromatographic peaks obtained during analysis, as illustrated in the chromatogram shown in FIG. 2.
Hence, what is needed is a back-flush injector 10 that allows for all of the sample introduced into the injector 10 to be properly injected into the pre-column and analytical column.
What is also needed is an injector 10 that is capable of back-flushing all of the sample remnant in the injector 10 after sample components of analytical concern have entered the analytical column.
According to one embodiment, a micro-machined back-flush injector that includes a sample inlet, an analytical column inlet channel, and a plurality of channels that connect the sample inlet and the analytical column inlet channel, wherein the plurality of channels include a fixed sample loop connecting a sample valve and a fore-flush valve in the injector.
According to another embodiment, a method of operating a back-flush injector that includes introducing a sample into the injector, injecting the sample into an analytical device, and purging substantially all of the sample from the injector.